As one of the important organic semiconductive materials, phthalocyanine-based compounds are widely used, as charge-carrier generation materials, in organic photoreceptors which are core components in xerography. In addition, they are widely used in various photoelectric systems such as organic solar cells, thin film transistors, organic light-emitting diodes, image sensors and the like (de la Torre G., Claessens C. G., Tones T., Chem. Commun., 2007, 2000). Generally, the unsubstituted phthalocyanine-based materials are superior in photoconductive properties to substituted phthalocyanine-based compounds; however, they have poor solubility and dispersibility in almost all kinds of solvents. The particles of unsubstituted phthalocyanines, prepared with conventional methods such as ball-milling or precipitation, have a particle diameter of about 20˜500 nanometers, which can not disperse and form stable colloidal particles in organic solvents in the absence of additives, largely restricting the applications of the phthalocyanine-based materials. On the other hand, the probability of charge separation for excitons occurring on the surface of phthalocyanine photoconductors is much greater than that in the bulk phase (Niimi T., Umeda M., J. Phys. Chem. B 2002, 106, 3657), while excitons diffuse a short distance in organic semiconductors, thereby, reducing the sizes of phthalocyanine-typed photoconductors and improving the specific surface areas thereof also enhance the charge carrier generating efficiencies thereof. We have reported a method for preparing phase-II vanadyl phthalocyanine (II-VOPc) and phase-II oxotitanium phthalocyanine (II-TiOPc, TiOPc has a molecular structural formula as shown in FIG. 1) photoconductor nanoparticles (Zhang X. R., Wang Y. F., Ma Y., Ye Y. C., Wang Y., Wu, K., Langmuir 2006, 22, 344; Chao W., Zhang X. R., Xiao C., Liang D. J., Wang, Y., J. Colloid Interface Sci., 2008, 325, 198), by which, the II-VOPc and II-TiOPc nanoparticles prepared can be dispersed in chloralkanes or chloroarenes to form stable colloidal solutions in the absence of additives. By dispersing such II-VOPc and II-TiOPc nanoparticles in an insulating resin, we have invented a single-layered organic photoreceptor (Wang Y. et al., Chinese patent No. ZL 03100821.6). As compared with the functional separation type multi-layered photoreceptors (such as Stolka, M et al., U.S. Pat. No. 4,265,990) and the single-layered organic photoreceptors composed of a small amount of phthalocyanines, a large amount of charge transfer materials and resins (such as KAWAHARA Emi et al., JP Patent 7291876-A), the positively charged single-layered photoreceptors consisted only of phthalocyanine photoconductor nanoparticles and insulating resins not only have the advantages of simple structure, low manufacturing costs, high stability, and wide applicability, but also can prevent a harm to the operator's health due to much ozone produced in the air when the surface is negatively charged; therefore, such a positively charged single-layered photoreceptor is one of major trends of the future organic photoreceptor development. The single-layered photoreceptor consisted of phase II VOPc or phase II TiOPc nanoparticles and insulating resins has a photoconductive mechanism, i.e., light-induced enhancement effect of electron tunneling, which is different from that of the functional separation type multi-layered photoreceptor and of the single-layered organic photoreceptor composed of a small amount of phthalocyanines, a large amount of charge transfer materials and resins. Such photoreceptors exhibit good photoconductive properties, however, there is a significant contradiction between the photoconductive sensitivity and dark decay rate thereof, that is, with the increase of the content of II-VOPc or II-TiOPc nanoparticles, both the photoconductive sensitivity of such single-layered photoreceptors as well as the dark decay rate thereof are remarkably increased. How to improve the photoconductive sensitivity of such single-layered photoreceptors while remarkably decreasing the dark decay rate thereof, this is a difficult problem needed to be addressed.
Much attention has been attracted to TiOPc due to diverse stacking patterns of molecules in its crystal and excellent photoconductive properties thereof (Law K. Y., Chem. Rev., 1993, 93, 499; Weiss D. S., Abkowitz M., Chem. Rev., 2010, 110, 479). Previous studies have shown that TiOPc has mainly four crystal forms, namely phase-I, phase-II, phase-Y, and phase-m, wherein, the phase-Y TiOPc (Y-TiOPc) exhibits the best photoconductive property (Fujimaki Y., Tadokoro H., Oda Y., Yoshioka H., Homma T., Moriguchi H., Watanabe K., Konishita A., Hirose N., Itami A., Ikeuchi S., J. Imag. Tech., 1991, 17, 202), and a charge-carrier photogeneration quantum yield of more than 90%, much higher than that of other phthalocyanine materials.
In conventional technologies for preparing Y-TiOPc, generally, the TiOPc in a sulfuric acid solution is firstly added to water to form a TiOPc precipitate, and then, to the dried (or undried) precipitate, a crystal form regulator is added to transform the crystal form so as to obtain the Y-TiOPc particles. However, this method is prone to result in incomplete transformation of the crystal form, on one hand, leaving much TiOPc in other crystal forms mixed in the resulting products, and on the other hand, molecular rearrangement tends to take place on the interfaces between particles contacted with each other in the precipitate during transformation of crystal forms to form rigid connections among particles or increase the particle sizes, decreasing the dispersibility of the particles, which make it difficult to prepare Y-TiOPc nanoparticles of small sizes. Chinese Patent No. ZL00,124736.0 reported a preparation method for Y-TiOPc nanoparticles having a particle diameter of 5˜20 nanometers, and Yang Lian-ming et al., using such Y-TiOPc nanoparticles as charge-carrier generation materials and hydrazone compounds as charge-carrier transport materials, prepared functional separation type multi-layered photoreceptors with excellent photoconductive properties. However, the Y-TiOPc nanoparticles prepared in accordance with this method can not disperse well in chloralkanes or chloroarenes to form a stable colloidal solution, and a lot of precipitates occurred after the dispersion stands for a certain period of time. The present inventors have tried to prepare single-layered organic photoreceptors by dispersing such nanoparticles into polycarbonates without adding any other charge-carrier transport materials; however, the resulting single-layered organic photoreceptors do not possess good overall photoconductive performances (see Comparative Example 2).
The present inventors have reported a preparation method for nanoscopic phthalocyanine-based organic semiconductor materials (Wang Y. et al., Chinese patent No. ZL 95117928.4). With this method, phthalocyanine nanoparticles having a particle diameter of 2˜8 nanometers may be prepared. However, the present inventors have demonstrated in the experimental results that this method can only give nanoparticles of II-TiOPc with mediocre photoconductive properties rather than Y-TiOPc nanoparticles (see Comparative Example 1). Based on the above, there is neither a report of Y-TiOPc nanoparticles with a particle diameter within the range of 2˜4 nanometers nor that of stable Y-TiOPc nanoparticle colloidal solutions formed by dispersing Y-TiOPc nanoparticles in solvents in the absence of the additives up to date. Therefore, reducing the sizes of Y-TiOPc nanoparticles and developing Y-TiOPc nanoparticles with high dispersion stability in organic solvents is a challenging subject that has important application prospects.